This invention relates to a capacitive sensor and more particularly, a center-mount capacitive sensor which has an overload protection mechanism.
Capacitance sensors are well known in the prior art. In some forms, capacitive sensors include a fixed element having a rigid, planar electrically conductive surface forming one plate of a substantially parallel plate capacitor. A deformable electrically conductive elastic member, such as a machined metal or metal foil diaphragm, forms the other plate of the capacitor. Generally, the diaphragm is edge-supported so that a central portion is substantially parallel to and opposite the fixed plate. In other forms, capacitive sensors are constructed with their capacitance plates connected at their centers. The electrically conductive surfaces forming the capacitor are insulated from the central connection and each other. Since the sensor has the form of a parallel plate capacitor, the characteristic capacitance of the sensor is inversely proportional to the gap, d, between central portion of the diaphragm and the conductive surface of the fixed element. In the case of a capacitive sensor for pressure, in order for there to permit a pressure differential across the diaphragm, the region on one side of the diaphragm is sealed from the region on the opposite side. Such capacitive sensor are also useful for sensing force, such as may be applied against the diaphragm relative to the support for the diaphragm edge.
For such sensors, the elasticity of the edge supported elastic member is selected so that pressure (or force) differentials across the elastic member in a particular range of interest, cause displacements of its central portion. These pressure differential-induced displacements result in corresponding variations in the gap, d, between the two capacitor plates, and thus in capacitance variations produced by the sensor. For relatively high sensitivity, such sensors require large changes of capacitance in response to relatively small gap changes. To achieve such sensitivity from unit to unit, nominal gap dimensions generally require that their component parts be manufactured to very close tolerances to establish the required dimensional relationships. In addition, the structure and materials must maintain those relationships over a useful temperature range.
One type of prior art sensor is disclosed in U.S. Pat. No. 3,859,575, assigned to the assignee of the subject invention. That prior art sensor, exemplified by the Model 209 transducer and transmitter, manufactured by Setra Systems, Inc., includes a tubular shaped member, an edge supported elastic member having a conductive portion, a flat electrode member with a central support means, a periphery conductive means, and an insulative means disposed between and connecting the periphery conductive means and the central support means. The elastic member is connected to the tubular member at its periphery. The flat electrode member is supported by a central stud passing through the elastic member and the central support means of the electrode member. A plane spacing washer is disposed between and separates the flat electrode and the elastic member by a precise distance. The periphery conductive means of the electrode member has a conductive surface opposite the conductive portion of the elastic member. Thus, a capacitor is formed by the elastic member and the flat electrode member. When force is applied to the elastic member, the distance between the conductive portion of the elastic member and the periphery conductive member of the electrode member will vary, and correspondingly, results in the changes of the capacitance of the capacitor.
In practice, a sensor of the above described types may be subjected to overloads which exceed the pressure measuring range of the sensor. Since these overloads may damage or destroy the sensing diaphragm, it is necessary to provide a mechanism to protect the sensing diaphragm when overloaded. One of conventional methods is to increase the thickness of the sensing diaphragm. Another general method is to use stronger material to make the diaphragm. Increasing the thickness or using stronger material often decreases motion of the diaphragm, and correspondingly decreases the sensitivity of the sensor.
It is an object of the present invention to provide an improved sensor having an overload protection mechanism.
It is another object of the present invention to provide an improved sensor with high sensitivity, and is relatively inexpensive and easy to manufacture.
The present invention is an improved capacitance sensor, adapted for high accuracy measurement of pressure having an overload protection mechanism. The sensor includes an elastic member, preferably a diaphragm, a plate or a beam. The elastic member disposed about a central axis of the sensor, and has a central region, a peripheral region, a first side, and a second side. The sensor comprises a support member supporting the peripheral region or edges of the elastic member. A post-extends from the central region of the elastic member along the central axis.
The sensor further includes an overpressure stop member fixedly coupled to (and supporting) the peripheral region of the elastic member or the support member. The overpressure stop member extends over the elastic member except for an aperture disposed about the central axis. The overpressure stop member has an inner surface facing the elastic member and an outer surface facing away from the overpressure stop member. In one preferred embodiment, the overpressure stop member is electrically conductive or has an electrically conductive portion on its outer surface.
The sensor further includes a first plate rigidly coupled (directly or indirectly) to the outer surface of the overpressure stop member. In one preferred embodiment, the first plate defines an aperture above the aperture defined by the overpressure stop member. The apertures are sized to allow passage of the post which extends therethrough. The first plate has a first surface, preferably facing away from the elastic member, comprising a first electrically conductive region.
The sensor further comprises a second plate extending from the post transverse to the central axis. The second plate has a second electrically conductive region opposite the first electrically conductive region of the first surface of the first plate. The opposite electrically conductive regions are electrically insulated from each other.
In one preferred form, the overpressure stop member and the first plate form an integral structure wherein the outer surface of the overpressure stop member and the first surface of the first plate are coincident. In another preferred form, the first plate is rigidly mounted to the outer surface of the overpressure stop member. In yet another preferred embodiment, the first plate and the overpressure stop member are discrete and spaced apart, both being rigidly coupled to the support member.
The post extends downwardly from a first end which is connected to the central region of the elastic member to a second end. The second plate extends radially and outwardly from the second end of the post. The first plate and the second plate are preferably substantially parallel, which there respective electrically conductive regions being oppositely each other.
The capacitive sensor may further include an electrically conductive path extending from the post via the elastic member to the first electrically conductive region of the first surface of the first plate. The second electrically conductive region of the first side of the second plate is electrically insulated from that path. In a preferred embodiment, the elastic member, the overpressure stop member, the first plate and the post are conductive, for example, all of them are made from metal, or where one or more of those elements is non-conductive, have an electrically conductive layers disposed thereon to form the electrically conductive path.
In one preferred embodiment, the inner surface of the overpressure stop member has a contour adapted to limit deflection of the elastic member caused by a differential pressure, or force, across the elastic member. Preferably, the contour defined by the inner surface of the overpressure stop member substantially conforms to a desired contour for by the elastic member short of its limits of elastic deformation, when the elastic member is deflected to the limiting contour defined by the overpressure stop member, the inner surface of the overpressure stop member can fully and uniformly supports the central region of the elastic member when the elastic member is overloaded.
The differential pressure can derive from a constant, controlled environment pressure being in contact with the first side of the elastic member and a pressure to be measured being in contact with the second side of the elastic member. The force can be a proof load or a burst load.
In a preferred embodiment, an electrode assembly is electrically connected to the first and second electrically conductive regions. That electrode assembly is adapted to indicate the pressure differential, or the force resulting in corresponding changes in capacitance. By way of example, where the electrodes of the sensor are coupled across a fixed inductor to form an oscillator, the frequency of the oscillator varies with the pressure-differential or force induced changes in capacitance of the sensor, and thus is representative of the pressure differential or force.
In one preferred embodiment, the elastic member further has a boss, preferably rigid, which is integral with, or affixed to, the elastic member in the central region, i.e., disposed about the central axis. The boss may be designed as a first point of contact with the overpressure stop member to minimize stresses over the elastic central region when the elastic member is overloaded.